JayCoin's motives were noble I'm sure but I never quite got the point of joining a pool that's nothing more than a node in p2pool. It defeats the whole purpose of p2pool which is DDOS-resistance.
P2Pmining's focus was 2-pronged...
First, it allowed smaller miners to still participate with p2pool without the big variance headache.
P2Pmining would earn shares of its own on the p2pool network, same as any miner, but it also ran its own share system, to further subdivide the p2pool payouts proportionally among the P2Pmining userbase.
This worked well for small miners that only had a couple hundred MH/s to work with.
The other focus was it also facilitated merge-mining for the userbase, saving them from having to run 4 or 5 different daemons at the same time on top of their own p2pool node.
IMHO, P2Pmining was nice to have. I usually had my miners set up to round-robin between my own p2pool instance, P2Pmining, and BTCGuild. What P2Pmining offered me at the time was that it was also merge-mining DVC, which I couldn't merge-mine locally as there wasn't a windows binary daemon for it, although there is one now.
As more ASICs hit the scene, P2Pmining would have become even more important for GPU miners, who are finding it harder to hold onto the shares they generate as their orphan rate climbs.
(the TRC p2pool was having this problem when I got fed up and segregated my own TRC p2pool network so it didn't touch the main network... some very high hash miners just kept orphaning 75-80% of my shares, sometimes a whole minute later, and there weren't any more than 7 or 8 connected nodes at the time. Latency of basically empty blocks and a small p2pool network, shouldn't have been an issue at all, but those high rate miners just overwhelm the lower rate ones... and I can't confirm it, but I also think one of them was intentionally ignoring shares of others, only building on their own, but that is beyond this topic)
P2Pmining just helped with scale. As the p2pool network difficulty climbs, P2Pmining was a way to still offer lower difficulty shares without having to have the whole p2pool network change to accommodate it.